Harmonic and sum-frequency generation of pulsed laser radiation in BBO,LBO, and KD*P

The optical properties of the nonlinear crystals lithium borate (LBO), barium borate (BBO) and deuterated potassium phosphate (KD*P) are compared for second and third harmonic generation of Nd:YAG laser radiation. In an experimental investigation the conversion efficiency has been measured as a function of the energy density of 8 ns long laser pulses, generated by a commercial Nd:YAG oscillator-amplifier system. In LBO and BBO the second harmonic generation saturates at an energy density of about 1.5 J cm^–2 at efficiencies of 55–60%. In KD*P comparable efficiencies (40–55%) require energy densities of 2–2.6 J cm^–2. Similar results are obtained for frequency tripling. In LBO and BBO saturated efficiencies of 20–25% are measured at an energy density of about 1.5 J cm^–2. In KD*P efficiencies of 20% are obtained at energy densities exceeding 2 J cm^–2. Besides for doubling and tripling of Nd:YAG laser radiation the phase-matching is calculated for frequency conversion of tunable laser light. The results demonstrate that in LBO and BBO phase-matched sum-frequency mixing of UV and infrared laser light generates tunable radiation at wavelengths as short as the transmission cut-off at 160 nm and 190 nm, respectively.     

Laser Tunable Source for the Red and Near UV Regions of the Spectrum

The spectral and lasing characteristics of a number of dyes for the red and near IR portions of the lasing spectrum in laser and lamp pumpings are investigated. Smooth tuning of a pulse-periodic lamp-pumped laser in the 660–720 nm range and generation of its second harmonic in the 330–360 nm region were obtained. As a result, the radiation range of the earlier developed tunable laser source has been extended in the red and near UV regions of the spectrum. The use of mixtures of dyes and proton-donating additives in working solutions has substantially improved the energy characteristics of this source.     

Third harmonic generation of CO2 laser radiation in AgGaSe2 crystal

Generation of third harmonic of CO₂ laser radiation has been obtained in a type-II, ϑ=57° cut 9 mm thick AgGaSe₂ crystal for the first time by sum-frequency-mixing of the fundamental with its second harmonic, the latter being obtained using another type-I, ϑ=55° cut 11 mm thick AgGaSe₂ crystal. The energy conversion efficiencies obtained for second harmonic and third harmonic generations are 6.3% and 2.4% respectively with the input fundamental pump power density of 5.9 MW/cm² only. The wavelength of the fundamental CO₂ laser radiation used for the generation of harmonics is 10.6 μm, P(20) line. A compact TEA CO₂ laser source has been built in the laboratory.     

Generalized Approximation of Strong Interaction of Waves in the Theory of High-Power Radiation Frequency Doubling

The approximation of strong interaction of waves is developed to analyze the generation of the second harmonic under the conditions of self-action. Based on this method, an approximate expression is obtained for the generation efficiency of the second harmonic with allowance for the effect of higher nonlinearities, depletion of pumping radiation, and linear phase detuning. The effect of phase detuning and change in the spatial distribution of the amplitude of the fundamental radiation on the efficiency of second-harmonic generation has been analyzed. Good agreement of the results of the developed approximate method with experimental data and numerical calculations is shown. The optimum conditions of the second-harmonic generation in a wide range of the laser system radiation intensities are determined.     

Generation of 6.8–7.7 µm radiation in lithium iodate

Generation of infrared radiation in ts-infrared transmission window (6.8–7.7 µm) is reported for the first time by non-collinear difference-frequency mixing of the Nd: YAG second harmonic and the same-pumped dye (Rh-610) laser radiation in a lithium-iodate crystal. The spectrum of polythene sheets was run with the generated radiation.     

Optimum conditions for the generation of the second harmonic of intense laser radiation

A generalized approximation of strong interactions of waves is suggested for the theoretical analysis of second-harmonic generation under conditions of self-action. On the basis of the method suggested, approximate solutions for the efficiency of second-harmonic generation are obtained with regard to the influence of higher nonlinearities, depletion of pumping radiation, and linear phase mismatch. The effects of the phase mismatch and the spatial distribution of the amplitude of the fundamental harmonic on the efficiency of second harmonic generation by intense laser radiation is analyzed. The results obtained with the approximate method developed are shown to be in good agreement with known experimental data and numerical calculations. Optimum conditions for second harmonic generation are determined in a wide range of laser radiation intensity and at different spatial distributions of the fundamental harmonic amplitude.     

Generation of lower and higher harmonics in different plasma media with small <Emphasis Type="Italic">Z</Emphasis>

The generation of lower (third) and higher harmonics of femtosecond laser radiation in plasmas produced by laser ablation of different targets with a small atomic number Z (B, Be, Li) has been investigated. The high (10⁻³) efficiency of third-harmonic generation was observed in plasma produced on the boron surface. Efficient third-harmonic generation was also observed in beryllium plasma using femtosecond pulses of Ti:sapphire laser radiation (λ = 790 nm) and its second harmonic (395 nm). We could tune the higher harmonics generation spectrum by tuning the crystal converter when using 395-nm radiation to be converted. It is shown that, in plasmas formed on targets with small Z, the conversion efficiency and limiting generated harmonic order depend on the delay between the ablation pulse and the pulse to be converted.     

Tunable distributed-feedback laser based on dye doped microporous quartz glass

The first distributed feedback (DFB) dye laser on the basis of microporous quartz glass (MQG) is reported. MQG consists of a specially processed quartz sample doped with a dye. As a pumping source a second harmonic YAG: Nd³⁺ laser with 12.5 pps repetition rate was used. Stable generation of the narrow line tuned within a spectral range of 562–584 nm and 633–663 nm with a linewidth of 0.04 nm was obtained. A high photochemical stability of the dye in the quartz matrix and reliability of MQG as an active medium for the tunable DFB laser were experimentally demonstrated.     

Tunable yellow–green laser based on second harmonic generation of LiF:F−2 in KTP

A yellow–green solid-state laser source based on frequency- doubling of a room temperature LiF:F–2 laser in KTP is reported. Second harmonic generation of free-running and prism-tuned pulsed lasers has been characterized. Tunable operation between 555 and 595nm was obtained, with a conversion efficiency of 7.5% for the free-running laser, and up to 4% for the prism-tuned configuration. The angular bandwidth of 60milliradians exceeded the theoretical value by a factor of 3.5. A flattened tuning curve in the visible region was obtained through enhanced conversion efficiency in the wings of the LiF:F–2 laser tuning range.     

Mode effects in second harmonic generation and their uses for laser stabilization

The influence of laser modes on second harmonic generation has been investigated using a three mode He-Ne laser operating at 633 nm as the source of the fundamental radiation. Mode locking which occurs when the laser modes are symmetrically placed relative to the centre of the gain profile enhances the second harmonic signal. This effect has been used to stabilize the frequency of the He-Ne laser to ± 1.5 MHz.     

Laser resonance external photoelectric effect in F 2 − color centers of LiF crystals

The laser-resonance two-step external photoelectric effect is revealed in the F ₂ ^– color centers of LiF crystals exposed to nanosecond laser pulses. The first photoionization step uses IR radiation tunable within the resonance absorption band of the color centers and the second step the second harmonic of a Nd:YAG laser. The photoionization cross sections of the F ₂ ^– centers are estimated.     

Conditions of Phase Matching for the Second Harmonic Generation with ZnGeP2 Crystals

The reasons for the discrepancy between the available experimental data on the phase-matching conditions for the second harmonic generation with nonlinear ZnGeP₂ crystals are elucidated. The calculated and experimental phase-matching angles agree for crystals at room temperature and low pumping radiation power. Simultaneous solution of the consistent shortened equations for the complex amplitudes and of the heat conductivity equation demonstrates that pumping by intense laser beams causes the angles phase-matching to decrease by 5–25° and the long-wavelength edge of phase matching for doubled frequency of CO₂-laser radiation to increase from 10.32 to 10.8 m. An excess decrease in the angles phase-matching by 1–3° is due to the asymmetry of the thermal processes caused by the birefringence.     

Peculiarities of self-action in second harmonic generation of laser radiation in nonlinear crystals

By using the generalized method of strong interaction of nonlinear waves and computer simulation, an analysis of the influence of self-action effects on second harmonic generation of an intense diverging picosecond pulse is performed. Using the approximation of strong wave interaction, an analytic solution is obtained for frequency conversion of the laser radiation taking into account the pump intensity depletion, the influence of the angular dispersion effect (ADE), and the higher nonlinearities. It is shown that the self-action effects, the ADE, and the linear phase mismatch can compensate for each other, increasing the efficiency of second harmonic generation. Optimum conditions for second harmonic generation in converging and diverging laser beams were found. An asymmetry of the angular dependence of the second harmonic intensity was experimentally observed and theoretically explained. The asymmetry is caused by the reverse energy conversion of the second harmonic into the pump by the influence of the ADE and Kerr nonlinearities.     

Photoinduced non-linear optical diagnostic of SiNxOy/Si111 interfaces

Anisotropic (elliptically polarized) photoinduced second harmonic generation (PISHG) in SiN_xO_y/Si1 1 1 films was proposed for contact-less monitoring of specimens with different nitrogen to oxygen (N/O) ratios. As a source for the photoinducing light, we used a nitrogen Q-switched pulse laser at wavelengths of 315, 337 and 354 nm as well as doubled frequency YAG–Nd laser wavelength (λ=530 nm). The YAG : Nd pulse laser (λ=1.06 μm; W=30 MW; τ=10–50 ps) was used to measure the PISHG. All measurements were done in a reflected light regime. We found that the output PISHG signal was sensitive to the N/O ratio and the film thickness. Measurements of the PISHG versus pumping wavelengths, powers, incident angles as well as independent measurements of the DC-electric field induced second harmonic generation indicate the major role played in this process by axially symmetric photoexcited electron–phonon states. The SiN_xO_y films were synthesized using a technique of chemical evaporation at low pressures. Films with thickness varying between 10 and 30 nm and with an N/O ratio between 0 and 1 were obtained. Electrostatic potential distribution at the Si1 1 1–SiN_xO_y interfaces was calculated. Comparison of the experimentally obtained and quantum chemically calculated PISHG data are presented. High sensitivity of anisotropic PISHG to the N/O ratio and film thickness is revealed. The role of the electron–phonon interactions in the dependencies observed is discussed. We have shown that the PISHG method has higher sensitivity than the traditional extended X-ray absorption fine structure spectroscopic and linear optical method for films with the N/O ratio higher than 0.50.     

Parametric fluorescence in A D P and K D P excited by a 2573 Å CW Pump

Temperature tunable, 90° phase matchable, parametric fluorescence has been observed in ADP and KDP crystals. The fluorescence was pumped by 2573 Å CW radiation obtained from a second harmonic argon-ion laser. Fluorescence was observed in ADP over the complete visible spectrum by temperature tuning through the range —12 to+40° C. The measurements demonstrate the usefulness of these materials for visible parametric oscillators. The values of d(n _2v ^e –n _v ^o )/dT deduced from these and other measurements are found to disagree with values in the literature.     

Indicatrices of the Radiation Intensity of Laser Plasma Formations in the Air

We have investigated the indicatrices of the visible and IR radiation of laser plasma formed under irradiation of cadmium, indium, and silicon in the air by radiation from a monopulsed neodymium laser with a power density at the irradiation spot of up to 12 GW/cm² on the first harmonic and up to 4 GW/cm² on the second harmonic. It is shown that the radiant intensity indicatrices have a prolate form depending on the target material, the spectral range of observation, and the power density of the acting laser radiation. The radiant intensity of laser plasma in the 0.3–4.2 m range is approximately proportional to the laser radiation power density and depends on the target material.     

A rich spectrum from potassium vapor on the 42 S 1/2−32 D 5/2 two-photon resonance

A rich emission spectrum in the wavelength range 300–600 nm, including 50 spectral lines, has been observed as a pulsed dye laser is tuned to the 4² S _1/2–3² D _5/2 two-photon resonance in potassium vapor. It is found that many kinds of mechanisms, including energy pooling, harmonic generation, quadrupole radiation, excitation transfer, and multiwave mixing etc., are involved in producing these lines. The process for each spectral line has been identified and is presented in tabular form.This project was supported by National Science Council of R.O.C. under the grant No. NSC 79-0208-M 009-21     

Generation of ultraviolet radiation with wide angular tolerance in cesium lithium borate crystal

Tangential phase-matching has been realised in cesium lithium borate (CLBO) crystal for the first time for the generation of fourth harmonic (266 nm) of Nd:YAG and third harmonic (226.7 nm) of a dye laser radiation by second harmonic generation and sum-frequency mixing with the angular tolerance as large as 22 mrad and 21 mrad respectively, over one of the interacting beams. An energy conversion efficiency of 15% for fourth harmonic generation is obtained with a 5.5 mm thick crystal and with the average pump powers only 170 and 70 mW. A set of Sellmeier dispersion equations for the CLBO crystal have also been formulated.     

The self-injected XeCl excimer laser

We present experimental and modelling results of the first self-injected excimer laser. The intracavity losses of a XeCl oscillator are properly modulated by a Pockels cell allowing generation, amplification and extraction of short laser pulses with selectable duration in the range of 1–12 ns, tailored temporal profile and peak power increment up to a factor of three. Longer output laser pulses, up to 100 ns, can be obtained by slicing the intracavity laser radiation without peak power increment. Laser output peak powers in excess of 2 MW have been obtained, with remarkable reproducibility characteristics.ENEA fellow     

Heterodyne performance of a quasioptical Schottky diode detector: Theoretical and experimental results

A detailed theoretical and experimental study of the heterodyne performance of a quasioptical Schottky diode detector is presented. The experimental results have been obtained by mixing the radiation from a FIR laser with the output of a 67–73 GHz Klystron. The heterodyne signal variation versus various parameters and its relation to the special case of two lasers mixing are described. The mixer characteristics are a NEP value of 2×10^–19W/Hz and a detector bandwidth of at least 9 GHz. Experimental evidence of harmonics generation of submillimetric frequencies at the diode junction is also presented.